Nondeforming electrode structure and electron discharge device



ar 7, 1951 L. P. GARNER NONDEFORMING ELECTRODE STRUCTURE AND ELECTRON DISCHARGE DEVICE Filed May 26, 1949 2 Sheets-Sheet l ENTOR.

LLOYD GARNER I March 27, 1951 1.. P. GARNER NONDEFORMING ELECTR E S CTURE AND ELECTRON DISC GE ICE Filed May 26, 1949 2 Sheets-Sheet 2 INVENT OR LLOYD P. GARNER ATTO EY Patented Mar. 27, 1951 NONDEFORMING ELECTRODE STRUCTURE ANn E Eo oN DISCHARGE. DEVICE LlQydP. Garner, Lancaster, Pa., assignor. to Radio Corporat en ai -America, a corporation of Dela- Application May 2 6, 194?, Serial No. 95,398 32 Claims, (Cl. 313-278) The present invention relates to electrode assemblies and more particularly to such structures which effectively eliminate objectionable defor-. mation or movement of the electrodes as well as an electron discharge device embodying the same. It is common knowledge that a limitation on electrode structures as wellas devices embodymg the same is deformation of the electrode structure during processin oroperation. In the main such deformation results from thermal expansion of the electrode and mount and constructions utilized heretofore have only been partially successful in overcoming this with the result that deviceshaving electrode structures therein, in particular electron discharge devices, have been seriously limited in merit' The problem presented by such deformation is aggravatedas the size of the affected parts is increased, nevertheless even inthe smallest of tubes it must be effectively dealt with if the maximum merit is to be obt ained. Thus, in order to alllow for expansion of tube parts which are processed or operate at elevated temperatures it has been the practice to rigidly fasten one end of the part permitting the other end to move freely in a suitable guide such as the top mica in receiving type tubes. Another arrangement utilizes a spring and hook as with V and W filamentary cathodes. In lar e power tubes both the cathode and grid structures are'often self-supporting basketlike welded assemblies.

Heretof ore, it has also been considered necessary to use such methods a welding, bolting,

clamping and the like to join active electrode membersto their lead-in conductors or supports to provide the required mechanical rigidity, electrical and thermal contact since the junctions between the electrodes and their supports must safely pass considerable heatin current in the case of self-heating cathodes or displacement current in the case of grid and anode electrodes which acquire considerable heat due to electron bombardment or heat radiation from the cathode.

All of theseconventional methods have objectional disadvantages and in particular leave much t be desired in the case of high power tubes which of necessity depend upon accurately maintained close spacings. In View of the tendency of vacuum cleaned surfaces to seize one another, metal to metal sliding fits must be relatively loose in order to work at all and hence are not suitable where accurate close spacings are critical. Furthermore, the use of insulation such as mica for a slidin support is undesirable for voltage breakdown and vacuum reasons. Conventional springs are not able to carry Without damage the heating or displacement currents required by the supported electrodes so that their use is undesirably restricted while a spring and hook support allows considerable movement unsuitable for long closely spaced elements. In the case of basketetype self-supporting structures,

directions inresponseto forces resulting from unequal expansion of the variouscomponent parts of 'thebasket and its supports. Such weaving about requires objectionably large inter-electrode spacings 'to avoid serious, changes; in electrical characteristics and even short circuits Thus, it is a principal object of, my invention to provide a simplified and improved meansfor mountin electrodes which is not subject to deleterious deformations during processing or Qp r ibn- I Another object is theprovision of an electrode structure wherein the electrod is free toe lon gate during temperature cycling and. yet rigidly maintains the same accuratel positioned 1 1? mg operation and processing. A, rt j t of, y i e ti n. s or ovide an electrode structure for a pluralityof accuj rarely. c os rpo' ition d ements a of whi s, nd end nt of ement of. the; oth rs. and eachisfree to elongate andcontract; 's i ianotherq i t is e prov ono an les.- trode assembly whereinthe active electrode may be quickly detached and replaced even after long periods of operation. i

Yet another objQCt is theprovisionof anelectron discha gfidevice wherein a plurality ofe electrode elements are accurately positioned in close array; eachelement being independent of move ment' of th others and free to elongate and contract without undergoing deformation.

or the purpose of illustration, my invention will be described. in connection, with a' triode type tube, however, itshouldfbe understood hal it is. not limited to the particular device shown;

In carrying my invention into effect I hook or clip the desired number of electrode elements be tweenconducting support members and maintain each, element in axial tension. At least one of the conducti suppor m mbe sl ovid w th flexibl supported mounting blocksv to which the electrode, elements are hooked or clipped and which are free to move in a direction parallel to the axis of elongation of the electrode clef ments without deforming the same,

The novel features that are considered char acteristic of thisinvention are set forth with parcuiarity in the appended claims. The'invention as well as additional objects and advantages thereof will be best understood from the following description when read in connection with the accompanying drawings, in which Figure 1 is an elevational view partly in section of an electron discharge device'embodying my invention;

Figure 2 is a side elevational view on an enlarged. scale. of my quick detachable electrode mounting;'

Figure 3 is a sectional. view through the line 3-3 of. Figure, 2 in the direction of the. arrows;

Figure 4 is a fragmentary sectional view of a flexible non-deforming electrode assembly constructed in accordance with my invention and showing another embodiment of my quick detachable mounting; and

Figure 5 is a fragmentary perspective view of such an assembly for a circular array of electrode elements.

For the purpose of illustration, my invention will be described in connection with a triode type electron discharge device, however, it should be understood that it is not limited to the particular device shown, a high power tube capable of a power output of the order of 500,000 watts and forms the subject matter of my co-pending joint application with William N. Parker and Willis E. Harbaugh, filed March 17, 1949, Serial Number 81,932, and which is assign-ed to the same assignee as my present application. As will be readily apparent, certain features of my invention are applicable to the mounting of electrodes where a certain degree of freedom of movement is necessary and desirable while all other movements must be restrained.

Referring now to the drawings wherein corresponding parts have been designated by identical numbers and to Figure 1 in particular, electron discharge device I is an internally water cooled triode having a cathode and grid, each of which comprises a circular array of discrete elements or bar-like sections indicated respectively at I l and I2, surrounded by a cylindric anode l3 and all are housed in a demountable evacuated envelope as shown.

'Each of the cathode elements or sections H is supported adjacent its upper ends by a flexible support means indicated generally at M supported from central conducting tubular support member which is connected to a copper diaphragm 24 by means of which it is connected electrically to terminal ring [6.

Still with reference to Figure 1, it is seen that the other or lower end of each cathode element or section II is inflexibly mounted in ring I! brazed to outer cathode conducting support l8 at the lower end of a beam former array. Conducting support member I8 is coaxial and concentric with conducting support l5 and has a lower portion of increased diameter which, below terminal ring 19, forms part of the exterior envelope of the tube. The upper portion of conducting support l8 which is in the active region of the device [9 has a plurality of grooves machined therein one for each of the cathode sections of which there are forty-eight in the tube illustrated. Said grooves are separated by lands which function as the beam former array and focus the electron stream emitted from each cathode section. It is apparent, therefore, that each cathode element is flexibly mounted at one end-to one conducting support member and infiexibly mounted at the other end to the other conducting support member. Further it should be noted that the conducting supports l5, I8 are rigidly connected by a mechanicaly strong insulating and hermetic seal 28.

Insulated from and supported on central conductor I5 is a hat shaped grid support member 2i having a peripheral flange at its lower extremity with slots and centering or locating V notches formed therein. There is one slot for each grid element [2 which are equal in number to cathode elements ll. Grid elements [2 each hook into grid support member 2| and are accurately positioned by means of the slots and V notches, as will be more fully described herein below, in such manner that each grid element is opposed to one of the lands 20 intermediate two adjacent cathode elements. Adjacent their lower ends, each grid element or section 12 is hooked or clipped into a separate flexible support means 22 which in turn are supported from a grid terminal rin 23.

The cathode, grid and beam former arrays form closely positioned, critically spaced units with the cylindric anode. In the tube illustrated the cathode and grid elements are bars approximately 8 inches in length. It is obvious that such a construction would be subject to objectionable deformation during processing and operation which would render it inoperative were the cathode and grid elements not mounted in such manner that they may freely expand and contract without undergoing deformation.

Referring now to Figures 2 and 3 there is seen my preferred form of quick detachable electrode connection used in connection with both the cathode and grid elements H, [2 of tube I0. I provided a V groove 25 and slot 26 in mounting blocks 33 connected to two cooperating support means such as support l4 and ring H or grid hat 21 and support 22. The slots and v grooves of these co-actin members are in registration one With the other and between alined pairs I hook or clip each of the electrode elements which are formed with a restricted portion 21 and mounting head 29 adjacent each end. The rod-like electrode elements each hook or clip into the alined V grooves and slots, contacting the same atpoints 30 located at the intersection of each V groove and slot.

As shown most clearly in Figure 1 such a V groove and slot mounting may be provided in conjunction with flexible supports l4 and 22 as well as inflexible supports I! and 21. The speciflc laminated feature of flexible supports l4 and 22 form no part of my present invention but are fully disclosed and claimed in the co-pending application of Willis E. Harbaugh filed May 26, 1949, Serial Number 95,442, assigned to the same assignee a this present application. However, I shall describe these laminated supports in sufficient detail in order that my present invention may be clearly understood. Laminated support 14 is made up of a plurality of centrally apertured annular thin copper laminations slotted to form a plurality of segments one for each electrode element to be supported therefrom. The laminations are bonded together by solid diffusion at their central portions to form a substantiall solid central mounting block 3| which is clamped to L shaped ring 32 which in turn is clamped to central supporting conductor 15. The laminations are also bonded by solid diffusion to form solid peripheral mounting blocks at the end of each segment which correspond to mounting blocks 33 (Figs. 2 and 3) and are machined to form V grooves 25 and slot 26. Similarly, each flexible and resilient support 22 is constructed from a plurality of rectangular strips of red hard steel which are bonded by solid diffusion methods to form mounting blocks. In this instance additional copper laminations 34 are provided to provide thermal and electrical conductivity. To the outer copper lamination a copper block 35 is connected as by solid diffusion. The V groove is formed in block 35 to avoid impairing the conductivity of copper laminations 34 while the whole is slotted. As fully pointed out in the above referred to copending Medal-8: 6

applications one such support is provided for each grid elementor. rod l2. As previouslynoted both ring I! and. gridhat 21* are. provided with V grooves and. slotswhichserve to anchor the other ends .Offthfi electrodes associated therewith.

In order to maintain. each electrode element in. axial tension. as. it elongates as. a result of thermal expansion resilient means are provided; For each segment: of flexible-support I4. a: coil spring 36 in a cylindric tube (shown by solid line in Fig. l)" is provided whichacts against, an insulating link 31 which in turn engages rocker arm 38. Rocker arm 38'- atone=end isfree to pivot in a groove in the lower arm of L shaped ring- 32 while its other end is bifurcated and engages in another V. groove in the underside of block 33. Coil springs 36 in their cylinders are arranged in circular array in an annular channel formed by conducting support l8 and seal 28. Grid support members 22 are each resilient and serve to maintaineach grid element l2 in axial tension without any'additional tensioning means.

Referring now to Figures-4 and 5 in detail,- 50 and- 50 are substantiallysimilar embodiments of another form of flexible non-deforming electrode mounting or-support; Both; supports 50 and 50' are shown in connection with a circular array of cathode elements and as part of tube It], being a modified version thereof; As will be noted the parts shown in fragmentary form in 'Figures 4 and 5 which are similar to those corresponding thereto in device I!) shown in. Figure 1 are designated by the same numbers with a prime added for convenience, Annular discs 43, 44 are a pair of identical centrally apertured discs having a plurality of slots 45 thereinforming a plurality of alined segments or straps 46, 41 rigidly connected by blocks 48 adjacent their peripheries. The inner or central portion of discs 43, 44' are rigidly connected by an annular block 49. As in the case of previously described supports l4 and 22, support 50 forms what may be termed a p-antographic system. When any block 48 is deflected vertically as viewed in Figure 4 and indicated by. dottedlines, the other end of support 50 being rigidly supported, the outer face 5] of the block, 48 sodeflected remains parallel to the axis of electrode 39 and therefore applies no bending forces to the electrode which may tend to deform the same. Straps 46, 41 are sufficiently flexible in the regions adjacent blocks 48 and 49 that motion axially. of electrode 39 is virtually unimpeded. On the other hand, motion at right angles to the axis of the electrode is restrained and substantially eliminated by column or edgewise beam-action of straps 46, 41.

When segments of support 40 flex as electrode elements 39 and expand and contract, deflected blocks 48 may move toward and away from block 49 in a predetermined manner. However, at any particular operating temperature of the electrode elements the positionof blocks 48 are stiflly maintained and makes possible the precise close spacings of the electrodes of an electron discharge device.

Discs 43, 44 and blocks 48, 49 are copper-joined by brazing through other suitable materials and connectionsv may be utilized. I have found that copper straps 48, 41: ,020 inches thick by 0.50 inches wide by 2.0 inches long and separated 0.50 inches to besatisfactory although the width and separation may be as low as ten times the thickness.

Support. 507 may be clamped, as i i fi se ofsupport [4, on an L shaped; ring 52 provided with a. spacer 53.. Lower arm or dance, of ring 52. extends outwardly under strap 41 and has an annular V grooveadjacent the periphery of its upper. surface-into which oneend of each folded annularspring 55,is seated. The upper portion of eachspring 5.5 is slotted forming a spring finger 56. for each. pair of straps 46,- 41 andxbears against the associated block 48. Thus, each. electrode isv independently tensioned, If desired, coilsprings 38, links 31 and rocker-arms 3.8:' may be utilized in place ofspring 55, substantially as shown in;Figure 1:.

Though the electrode elements maybe connected to blocks 48 by conventional methods, I prefer to use the quick detachable connections previously described hereinabove and in particular the form shown in Figures Zand, 3. Tomount electrode 39 I provide a hole in each block 48 at anoblique anglewhile lower ring I1 isv provided with. complementary. holes, each being in alinement with. the hole in corresponding block 48 Ends, 4]; of electrode 39-; are turned back to form. mounting heads or hooks which seat inthe corresponding holes inblocks 4-8 and lower ring H. As indicated; I prefer to brazeor otherwise connect a copper block 33f having a V groove andslot toface 5110f each block48sto receive the upper heads 20 of electrode elements. Hz. The other or, lower mounting heads of electrode elements are hooked intoaslot. and V-groove formed in aring identical to ring I,1.

Where a greater degree of individualfreedom for the electrode elementsis; desired 1: use rectangular straps, theupper straps being connected only to the lower strapsin alinementtherewith and not to the-adjacent upper straps. Similarly, each of the lower straps is not connected toadjacent lower straps. Such. individual, flexible sup.- port members may be arranged in circular or straight arrayszas desired. However, supports-50 and 5.8 are suitable for an array of electrode elements having similar thermal expansions most of which is taken up by the portions, of" discs 43,44 adjacent block- 49. Individual, variations in expansion are then accommodated by individual straps 46,41;

From; the foregoing it is apparent that I have provided an electron discharge device having an electrode structure which greatly simplifies assembly thereof which permits ready replacement of electrode elements even after long periods of operation, and which permits accurate close spacings, of electrode elements free from deformation caused by thermal expansion. Furthermore, the electrode elements may be pre-treated and pre-processed such as coatingor flashing in controlled atmospheres priorto mounting. It should be clearly understood that the slots 26 aswell as the holes corresponding thereto shown in Figure 4 are so dimensioned as to provide sufiicient clearance between the electrodes and the mounting blocks to avoid the possibility of undesired intermittent contacts occurring. It is further apparent that each electrodeis firmly positioned and is not free to wobble. ,Thus, points. of contact are limited to points 30 (Figure 2) and 5| (Figure 4) My invention is not limited to copper mounting blocks and tungsten electrodes as illustrated but is equally applicable to other metals. Though I preferably select metals which will not undergo solid diffusion at the processing or operating temperatures to which the device may be subjected in order to insure that the electrode may be readily detached. When the electrodes will withstand relatively small tension forces, I provide relatively small contact points in order to insure high unit pressure at the contact points. It is obvious that if desired, flexible mountings may be provided for each end of the electrode elements instead of only for one end as shown. In addition other tensioning means may be utilized in connection with my quick detachable mounting than those shown without departing from the scope of my invention. For example, the electrode elements may be of the folded or hair pin type hooked into their lead-in conductors and tensioned at their central portion by means of a spring as in tube type 207. Therefore, while my invention is subject to obvious modifications by those skilled in the art, it is intended to cover all such modifications as come within the scope of the appended claims.

I claim:

1. An electrode structure comprising an electrode, a conducting support member quickly detachably connected to said electrode adjacent one end thereof, another conducting support member connected to said electrode adjacent the other end thereof, and resilient mean maintaining said electrode and said support members in good electrical contact.

2. An electrode assembly, comprising a conducting support member, a second conducting support member insulated from said first mentioned support member, and an electrode detachably interlocked at each of its ends with one of said support members.

3. An electrode assembly, comprising an elongated electrode having a hook formed adjacent each of its ends, a conducting support member in quick detachable engagement with one of said hooks, and a second conducting support member in quick detachable engagement with the other one of said hooks.

4. An electrode assembly comprising, an electrode, means for supporting said electrode, said means including a flexible support quick detachably connected to said electrode adjacent one end thereof, said means further including resilient means for maintaining said electrode in tension.

5. An electrode assembly, comprising an elongated electrode, flexible means supporting said electrode adjacent one end thereof, said flexible means including a pair of flexible straps rigidly connected together adjacent each of their ends, means for supporting the other end of said electrode, and a conducting support connected to at least one of said previousl mentioned means.

6. An electrode assembly, comprisin an elongated electrode having a hook formed adjacent each of its ends, a conducting support member having a hole formed therein, one of said hooks extending into said hole and in quick detachable engagement with said support member, a second conducting support member having a hole formed therein, the other of said hooks extending into said last mentioned hole and in quick detachable engagement with said second support member, and resilient means for maintaining said electrode in axial tension.

7. An electrode assembly, comprising an elongated electrode having a head formed thereon adjacent each of its ends, support means for each end of said electrode and having slots formed therein through which said electrode extends with each of its heads in quick detachable engagement with said support means, and said support means further includin resilient means for maintaining said electrode in axial tension.

8. An electrode assembly, comprising an elongated electrode having a head formed thereon adjacent each of its ends, support means for each end of said electrode and having slots formed therein through which said electrode extends with each of its heads in quick detachable engagement with said support means, each of said means having a V groove formed therein to seat said heads, and said support means further including resilient means for maintaining said electrode in axial tension.

9. An electrode assembly, comprising an elongated electrode, conducting support means for each end of said electrode, a pair of substantially parallel elongated straps in spaced apart relation and rigidly connected adjacent one end to the support means, means rigidly connecting the other ends of said straps, the portion of said strap intermediate their ends being unconnected and free to flex, said last mentioned means being connected to said electrode adjacent said one end thereof, and said electrode also being connected adjacent its other end to said support means.

10. An electrode assembly, comprising an elongated electrode, flexible means supporting said electrode adjacent one end thereof, said flexible means including a pair of flexible straps rigidly connected together adjacent each of their ends, an means for supporting the other end of said electrode.

11. An electron discharge device, comprising an elongated electrode, conducting flexible means supporting said electrode adjacent one end thereof, said flexible means including a pair of flexible straps rigidly connected together adjacent each of their ends, and means for supporting the other end of said electrode.

12. An electron discharge device, comprising at least two co-acting electrodes, support means supporting said electrodes, and at least one of said electrodes being in quick detachable engagement with its sup-port means and only engaging the same.

13. An electron discharge device, comprising at least one electrode, support means supporting said electrode, and said electrode being elongated and only in quick detachable interlocking engagement adjacent each end thereof with its support means and only engaging the same.

14. An electron discharge device, comprising at least two co-acting electrodes, support means for said electrodes, at least one of said electrodes bein elongated and having mounting heads formed adjacent each of its ends, and said mounting heads being in quick detachable interlocking engagement with said support means.

15. An electron discharge device comprising at least two co-acting electrodes, support means for each of said electrodes, at least one of said electrodes being elongated and having mounting heads formed adjacent each of its ends, said support means including a pair of spaced apart flexible straps, said straps being connected to said support means adjacent one end thereof, a block rigidly connecting said straps adjacent the other end thereof, the portions of said straps intermediate the ends thereof being free to flex, one

of said heads being interlocked with said block and the other of said heads being interlocked with said support means, and said support means further including resilient means for maintaining said elongated electrode in axial tension.

16. An electron discharge device, comprising an electrode having a plurality of spaced apart elements, support means for said electrode and including flexible means forming the sole connection and support between said support means and one end of each of said electrode elements, said flexible means also serving as the electrical current path to said end of each of said electrode elements, and said flexible means separately and independently tensioni'ng each of said electrode elements.

17. An electron discharge device, comprising an electrode having a plurality of spaced members and a mount for said electrode, said mount including mutually independent flexible means connected to each of said electrode members and forming the sole connection between one end of each of said members and said mount, and a plurality of mutually independent tensioning means one for each of said members and resiliently engaging said mount and said flexible means and maintaining said members under tension.

18. An electron discharge device, comprising an electrode having a plurality of discrete spaced elements, a current lead for said electrode, and flexible support means supported on said current lead and separately and independently supporting and tensioning each of said electrode elements, said flexible support means forming the sole support and connection to said current lead for at least one end of each of said elements.

19. An electron discharge device, comprising an electrode having a plurality of discrete sections, a current lead for said electrode, and means adjustably supported on said current lead and separately independently supporting and tensioning each of said electrode elements.

20. An electron discharge device, comprising an electrode having an array of spaced apart elements, current lead-in means, flexible means connected to each of said elements adjacent an end thereof and to said current lead-in means, means connected to said current lead-in means and in engagement with each of said elements at a point remote from said flexible means, and resilient means supported by said current lead-in means and in engagement with said flexible means.

21. An electron discharge device, comprising an electrode having an array of spaced apart members, a pair of supporting conductors in spaced apart relation, flexible means quick detachably connected to each of said elements and connected to one of said supporting conductors, resilient means in engagement with said flexible means, and each of said members bein connected to the other of said supporting conductors at a point remote from said flexible means.

22. An electron discharge device, comprising an electrode having an array of discrete spaced elements, coaxial and concentric supporting conductors in spaced relation, flexible means connected to each of said elements and to one of said supporting conductors, resilient means on the other of said supporting conductors and in engagement with said flexible means, each of said elements being connected to the other of said supporting conductors at a point remote from said flexible means.

23. .An electron discharge device, comprising a filamentary cathode having a circular array of a plurality of elongated spaced elements, inner and outer tubular coaxial conducting supporting members, a circular array of a plurality of flexible means joined together at one end thereof and mutually independent at the other end where each one supports one of said cathode elements, said flexible means being supported on said inner member, each of said elements being connected to said outer member at a point remote from said flexible means.

24. An electrode assembly, comprising an elongated electrode having a mounting head formed thereon and integral therewith adjacent at least one of its ends, and support means for said electrode and having a slot formed therein through which said electrode extends with its mounting head in quick detachable engagement with said support means.

25. An electrode assembly, comprising an elongated electrode having a mountin head formed thereon and a portion of reduced cross-section merging into said mounting head, and support means for said electrode, said mounting head being in quick detachable engagement with said support means.

26. An electrode assembly, comprisin an elongated electrode having portions of reduced crosssection adjacent each end thereof and mounting heads formed thereon merging with each of said reduced portions, and support means having slots formed therein through which said reduced portions extend with said mounting heads in quick detachable engagement with said support means.

27. An electrode, comprising an elongated element having a portion of reduced cross-section and a mounting head formed thereon merging with said portion of reduced cross-section.

28. An electrode, comprising an elongated element having portions of reduced cross-section and mounting means formed thereon each merging with one of said portions of reduced crosssection.

29. An electrode, comprising an elongated barlike element having portions of reduced crosssection adjacent each end thereof and mounting heads formed therein merging with each of said reduced portions.

30. An electron discharge device, comprising an electrode, said electrode including mounting means, and conducting lead-in means only quick detachably connected to said mounting means, said electrode being solely connected to said conducting lead-in means.

31. An electron discharge device, comprising an elongated electrode having mounting heads formed therein, and conducting lead-in means for said electrode and in quick detachable engagement with said mounting heads.

32. An electron discharge device, comprising a plurality of spaced cathode elements and mounting means including a notch and groove for each of said elements, at least one end of each of said elements being provided with temperature compensating means comprising a reduced shank portion and a mounting head formed adjacent the end thereof, said shank portion being inserted in said notch and said head resting in said groove.

LLOYD P. GARNER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,560,691 Houskeeper Nov. 10, 1925 1,572,721 Houskeeper Feb. 9, 1926 1,944,190 Mouromtseff Jan. 23, 1934 2,186,127 Samuel Jan. 9, 1940 2,229,957 Crawford Jan. 28, 1941 2,380,502 Clark et a1 July 31, 1945 2,410,993 Nelson Nov. 12, 1946 

